Experimental research work
A new experimental device, named the Gas Fracture Visualisation Rig (GFVR), was developed during the project. This device enables the visualisation of the formation of Discrete Gas Flow Pathways (DGFP) as gas is injected into the centre of a thin layer of saturated clay. This clay layer is compressed in oedometric conditions between two thick glass discs. The GFVR allows for the control of several parameters: the water back-pressure at the sample perimeter, the relative displacement between the glass discs, and the compression force applied to the clay. The experimental setup is completed by gas and water circuits connected to two high-pressure, high-precision fluid pumps, which enforce the targeted fluid conditions at the injection and back-pressure filters of the device. To improve the visualisation of DGFPs, the GFVR is placed within a ‘black box’ equipped with a backlight underneath and a high-resolution camera on top. This setup enables the detection of features invisible to the naked eye.
The device has undergone a series of 13 preliminary tests, during which a detailed testing protocol was developed, and minor design and manufacturing flaws were corrected. Currently, the device is fully operative and ready to undertake systematic studies on the factors controlling the formation and disappearance of DGFPs in saturated clay.
The preliminary tests conducted to verify GFVR, along with the numerical simulations performed using the numerical model developed within the project, have enabled the researcher to identify several factors controlling the development of DGFPs in the device. These factors include the loading history, the gas injection rate, and the back-pressure level. Based on these findings, an experimental campaign was designed to systematically investigate these factors.
Numerical modelling work
A novel Pneumo-Hydro-Mechanical (PHM) Finite Element model designed to simulate gas migration in saturated clay samples of a few centimetres was developed during the project. The model employs continuum elements to represent the mechanical and flow processes in the bulk clay material, while zero-thickness interface elements are utilised to simulate existing or induced discontinuities (cracks). To accomplish this, a new triple-node PHM interface element was introduced. The model performance was verified through a series of synthetic benchmark examples, demonstrating its capability to replicate observed PHM mechanisms that lead to crack propagation due to gas pressure (gas fracturing). Furthermore, the model facilitated scoping simulations to aid in designing the GFVR and selecting the factors for exploration in the experimental campaign. This necessitated the development of additional numerical tools to accurately account for the boundary conditions of the clay sample within the GFVR.
The model has been applied in analysing laboratory gas injection tests on Boom clay using two different experimental setups. Through these analyses, the model proved its utility in offering new insights into the mechanisms governing the formation of gas cracks in gas migration tests.
Dissemination and Exploitation
Throughout the duration of the project, concerted efforts have been made to disseminate progress and outcomes to both specialised and non-specialised audiences. These efforts encompassed participation in in international conferences and workshops, papers in international journals, research reports, open access experimental datasets, research seminars, web articles, social network posts, and video clips. Parallel to these dissemination activities, the project has pursued engagement with end-users spanning a diverse array of sectors including academia, government bodies, technical support organisations, and industry.
